Basically, there's not a lot in the way of money making in the game, so you have to capture enemies to get new stuff. Using the capture command - only available when you have a higher build then your target, or if the target is unequipped - has you do weakened attacks to reduce an opponent's HP to zero. If it happens, you get the opponent as if you rescued them and can strip them of their stuff. Captured enemies with no weapon are automatically captured.

Really hard. Not helping anything is the fact that they removed the fixing weapons thing from Geneology of the Holy War, meaning that the really really good weapons you get you have to be liberal in usage.

Of course, this goes for every Fire Emblem game ever (except for, obviously, Geneology of the Holy War (and Gaiden, as it didn't have weapon uses)), which is stupid.